1. Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.

2. In a large bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix until well combined.

3. In a separate bowl, whisk together egg, milk, vegetable oil, and vanilla extract.

4. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.

5. Stir in the boiling water until smooth (batter will be thin).

6. Divide the batter evenly among the 12 cupcake liners, filling each about 2/3 full.

7. Bake for 18-20 minutes, or until a toothpick comes out clean.

8. Let the cupcakes cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

For the Filling:

1. In a medium bowl, combine marshmallow fluff and vegetable shortening. Mix until smooth.

2. Gradually add powdered sugar and vanilla, beating until fluffy.

3. Transfer the filling to a piping bag or zip-top bag with the tip cut off.

For the Ganache Topping:

1. Place chocolate chips in a heatproof bowl.

2. Heat heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil).

3. Pour the hot cream over the chocolate chips, let sit for 2 minutes, and stir until smooth. Stir in vanilla extract.

4. Let the ganache cool slightly but remain pourable.

Assembling the Cupcakes:

1. Once cupcakes are cool, remove a small portion from the center of each cupcake to create a hollow space.

2. Pipe marshmallow filling into the center of each cupcake.

3. Spoon or drizzle ganache over the top of each filled cupcake.

4. Optionally, use the remaining ganache to create a swirl on top using a piping bag or fork.

5. Let the ganache set before serving.